Yea 680 is enough wattage. I personally would go with the GTX 260, since i'm more a fan of powerful single card solutions. I'm not 100% sure, but i'm thinking a 260 is better than 2 9800's in SLI anyway, although i could be way off base.

What make and model PSU are you going with?

Actually i think a 260 and the 9800GT SLI trade blows from what i can find on the net. Someone with a more personal approach could explain a little better maybe.

680 is plenty for 9800s in SLI... but an OCed core 216 will perform about equal to the 9800GT in synthetic tests, but as always, the single card solution performs better in games not only because of the multi GPU support issues in most games (ie non-native support), but also because the monolithic structure makes the single card solution transfer data faster, which can become a limiting factor at high details/resolutions

1 gtx 260 your lowest fps will be higher with the single card your max fps may not be as high tho, but your lowest fps will be higher with a gtx 260. if i was you i would try and fidn a set of used 9800gtx or 8800gts 512mb for sale in a pair and use them for sli.